Sets the number of threads performing background operations in table engines (for example, merges in [MergeTree engine](../../engines/table-engines/mergetree-family/index.md) tables). This setting is applied from `default` profile at ClickHouse server start and can’t be changed in a user session. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
Before changing it, please also take a look at related [MergeTree settings](../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-merge_tree), such as `number_of_free_entries_in_pool_to_lower_max_size_of_merge` and `number_of_free_entries_in_pool_to_execute_mutation`
Before changing it, please also take a look at related [MergeTree settings](../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-merge_tree), such as `number_of_free_entries_in_pool_to_lower_max_size_of_merge` and `number_of_free_entries_in_pool_to_execute_mutation`.
